1. Field of the Invention
This invention relates to a food container containing processed or precooked foods, which are subjected to microwave heating, or chilled or frozen foods, which are subjected to microwave cooking or heating, and particularly to that allows for uniform temperature increases and uniform heat distribution in filled precooked foods (the aforesaid foods are hereinafter referred to as "precooked foods") when heated with a microwave oven.
2. Background of the Art
Precooked foods, which are easily used simply by heating with a microwave oven when served, are commercially available. Such precooked foods include gratin, lasagna, rice with curry, or the like, which is packed into a container for microwave heating. Heretofore, the containers for microwave heating normally have been made of a microwave transmittable material. However, the containers are associated with problems in that, upon heating with a microwave oven, the temperature of a portion of the precooked food near an inner side wall of the container is high while that of a center portion of the precooked food is low, and in that the temperature near the upper surface of the precooked food rises quickly while the temperature near the bottom of the container rises slowly.
In view of the above problems of unevenness in temperature and heating rates, various types of containers, which alleviate uneven heating when heating the containers containing precooked foods with a microwave oven, have been developed.
For example, Japanese Patent Publication No. 59-6789 discloses a food package for heating or cooking precooked foods using a hot air convection type of home-use microwave oven, which food package comprises a bottom, a circumferential side wall having an upper edge outwardly curved, and a tray having circumferential rims extending horizontally, wherein a circumferential surface of the side wall of the container, or at least an inner surface thereof, or an upper portion of the rims and a wall portion adjacent to their corners are made of or coated with a microwave reflective material or microwave non-transmittable material, and wherein the bottom is made of a microwave transmittable material, or is detachable.
Further, Japanese Patent Laid-open No. 52-112150 discloses a food container for microwave heating, comprising: a body having a bottom wall, a side circumferential wall, and projections projecting from the bottom wall and forming a plurality of segments; and a lid having a top wall made of a microwave transmittable material, a circumferential side wall, and projections projecting from the side circumferential wall and the top wall, wherein a plurality of individual segments are formed, separately from each other, by combining the body, the lid, and the respective circumferential wall and projections, and wherein the circumferential side wall and the projections of the lid extend to the interior of the segments of the body so as to fit into the circumferential side wall and the projections of the body. This patent application also teaches that selective heating suitable for a food stored in each individual segment can be conducted by selectively locating microwave-transmittable portions and microwave non-transmittable portions formed on the outer surface of the container, using a metal foil or ink, which does not transmit microwaves, on the outer surface of the body and the lid, thereby preventing transmission of microwaves therethrough.
As described above, various types of food containers for microwave heating have been developed. However, the container disclosed in Japanese patent publication No. 59-6789 can prevent an extreme temperature rise on the inner side of the circumferential walls or the corners; however, since, in the container, the side circumferential walls or the corners formed by the rims and the container are simply coated with a microwave non-transmittable material, a temperature rise in the center of the container, especially in the center of a frozen food adjacent to the bottom, cannot be controlled effectively. Thus, the container has a problem in that it is difficult to uniformly heat a precooked food at an even temperature rise.
On the other hand, the container for microwave heating disclosed in Japanese Patent Laid-open No. 52-112150 is designed for heating different foods stored in the respective segments by differentiating heating manners depending on the food; however, since heating of the respective segments is conducted by passing microwaves through a microwave transmission hole having a limited area located in the center, the temperature rises quickly in the center, but the temperature rises slowly at the circumferential wall and the corners. Thus, nonuniformity of heating cannot be avoided.
As described above, heretofore, any containers containing a frozen food for microwave heating do not allow uniform heating in the frozen food placed evenly in the containers, thereby limiting embodiments or applications of the containers.